The concept of monogamy evolved so that bloodlines could be passed down from father to son. If women were not forced by society to be monogamous, there was no way for men to know who the father of the child was.

It is only relatively recently that men are expected to be monogamous too.

I am a woman in a heterosexual relationship and we are nonmonogamous. It has been wonderful for our sex life and our marriage. I would encourage more straight couples to try it!